This application claims priority from Korean Patent Application No. 10-2004-0004562, filed on Jan. 24, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an edge light type backlight unit, and more particularly, to a backlight unit having an improved light-reception portion.
2. Description of the Related Art
Typically, a liquid crystal display (LCD) device is one of light-reception flat panel displays and needs a separate light source to produce an image since the LCD itself does not emit light. To this end, a backlight unit is located behind the LCD device and emits light. The backlight units are used in LCD devices as well as in surface light source systems such as illuminating signs.
Backlight units are classified into direct light type units and edge light type units according to the position of a light source. For a direct light type unit, a lamp disposed beneath a LCD panel directly emits light onto the LCD panel. For an edge light type unit, a lamp located on an edge of a light guide panel (LGP) emits light onto the LCD panel via the LGP.
An edge light type unit may include a linear light source or a point light source as an illuminant. Representative examples of the linear light source and point light source are a cold cathode fluorescent lamp (CCFL) having two electrodes at opposite ends within a tube and a light emitting diode (LED), respectively.
While a CCFL provides bright white light, high brightness, high luminous uniformity, and a large area design, it operates in response to a high frequency alternating current (AC) signal and has a narrow temperature range. In contrast to the CCFL, an LED exhibits lower brightness and luminous uniformity. However, the LED operates according to a direct current (DC) signal, has a long life span and a wide temperature range and minimizes the thickness.
The LGP, which is used for an edge light type unit, converts light emitted by a light source through its edge into surface light and outputs light in a vertical direction. The LGP has a scattering or holographic pattern formed by a printing or machining method for conversion into the surface light.
FIG. 1 is a schematic perspective view of a conventional edge type backlight unit using a point light source, FIG. 2 is a cross-sectional view of FIG. 1, and FIG. 3 is a graph illustrating an azimuth angle of an LED. Referring to FIGS. 1-3, three LEDs 20, which are point light sources, are disposed along one edge 11 of a LGP 10. The LGP 10 has a holographic pattern 30 at the bottom thereof, which allows light emitted by the LED 20 to be directed onto a light-emission surface 12. The LED 20 emits light toward the edge 11 in an angular range between −90° and +90° about an optical axis as shown in FIG. 3. In this case, an azimuth angle at which light having intensity Imax/2 that is half the maximum light intensity Imax is emitted is called a forward half maximum (FWHM) angle. A typical LED has a FWHM angle between about −45° and +45°.
The light emitted from the LED 20 is incident on the LGP 10 through the edge 11. The holographic pattern 30 with a diffractive grating structure and which is oriented in a predetermined direction converts the incident light into surface light and diffracts the surface light onto the light-emission surface 12.
Furthermore, as the range of azimuth angles of light being incident on the holographic pattern 30 decreases, uniformity of brightness over the light-emission surface 12 increases. Uneven brightness over the light-emission surface 12 makes a screen appear mottled. For example, while a variation in brightness of about 0.9 within a narrow range of about 1 cm may be detected as a stain on the screen, a slow variation in brightness of about 0.8 between the screen's center and corners may not be detected as a stain. Thus, at least brightness uniformity over 0.8 is required to prevent a stain on the screen. To achieve a better quality image, brightness uniformity over 0.9 is needed.
FIG. 4 is a plan view of FIG. 1, which illustrates distribution of light output through the light-emission surface 12. Referring to FIG. 4, the light-emission surface 12 is divided into a first region 12a, a second region 12b, and a third region 12c, which are disposed in a direction away from the edge 11. The distribution of light output from each of the regions 12a, 12b, and 12c is as shown in FIG. 4. That is, the second and third regions 12b and 12c have wider distributions of outgoing light than the first region 12a. 
FIG. 5 is a graph of brightness vs. exit angle represented by a FWHM angle. Three curves C1-C3 represent brightnesses of light output from the first region 12a, the second region 12b, and the third region 12c, respectively. As is evident by FIG. 5, the brightness on the first region 12a is greater than those on the second region 12b and the third region 12c. The first region 12a has a FWHM angle of 20°/20° while the second region 12b and the third region 12c have a FWHM angle of 20°/35° where the values before and after the slash “/” denote FWHM angles in X and Y directions in FIG. 4, respectively.
The difference in brightness on each region 12a, 12b, or 12c arises because the range of azimuth angles of light incident on the holographic pattern 30 is narrower in the first region 12a than in the remaining regions 12b and 12c. That is, in the second region 12b and the third region 12c, light having a wider range of azimuth angles after being reflected many times is incident on the holographic pattern 30. The brightness uniformity decreases as an azimuth angle of light emitted by the LED 20 on the LGP 10 increases.
As shown in FIG. 2, the back light unit further includes two prism sheets 15 and 17 disposed on the LGP 10 so that light obliquely incident on and output from the light-emission surface 12 propagates in the same direction as light diffracted by the holographic pattern 30. Thus, the conventional backlight unit suffers from low optical efficiency, large number of assembling steps, and high manufacturing costs.